Preheat oven to 350º and grease and flour a 9-x-13" cake pan. Prepare yellow cake mix according to box instructions. Pour batter into prepared pan and bake until a toothpick inserted in the center comes out clean, about 25 minutes. Let cool 10 minutes.
Meanwhile, prepare pudding: In a small bowl, whisk together pudding packets and milk until thick. Set aside.
In another large bowl, beat cream, sugar, and vanilla until stiff peaks form. Set aside.
Using the bottom of a wooden spoon, poke holes all over the cake. Spread pudding mixture on top and top with a layer of banana slices. Spread whipped cream on top and sprinkle all over with crushed and whole Nilla Wafers and bananas.
Slice and serve.